Kale and leafy greens are enlivened by a squeeze of citrus; a salad is improved immeasurably by olive oil dressing and a morning bowl of cereal given a lift by a glass of orange juice on the side. These are classic combinations - but they are also examples of a new area of interest in nutrition, food 'synergy'.
The idea is that combining certain foods can help us better absorb nutrients: "It's quite a new science," says Fiona Hunter, a nutritionist at Discover Great Veg. "The focus has always been on single nutrients and foods, but recently people have started to discover that pairing foods can enhance absorption.
"If you have your breakfast cereal with a glass of orange juice, you're going to absorb much more iron from that cereal, because the Vitamin C alters the structure of the iron, changing the chemical formulation slightly, making it easier for the body to absorb."
Pairings already known to nutritionists include vitamin C and iron, calcium and vitamin D, fat-soluble vitamins like vitamins A and D with fats, and calcium with vitamin D.
Registered associate nutritionist Katy Mortimer highlights that eating vitamin D (found naturally in fatty fish, such as tuna, mackerel, and salmon, cheese and egg yolks) for example, can boost calcium absorption in a meal by 30-40 per cent. Many breakfast cereals are fortified with vitamin D, so eating them with milk or calcium-boosted milk alternatives will help.
This may all sound rather complicated, but many recommended matches naturally go well together. Adding some lemon to kale or cavolo nero, for instance, makes the iron more easily absorbed. Leeks and cheese is another traditional combination - and recent research shows the inulin in leeks helps boost levels of 'good' bacteria in the digestive system, which can in turn help our bodies process absorb calcium.
Whether this is down to some inherent knowledge built up throughout evolution is yet to be studied. However, it begs the question for Hunter: "did our parents and grandparents know something that we didn't, without having the science to support it?"
For both nutritionists, a key takeaway is to move us away from concentrating on single nutrients. "Focusing on individual nutrients can sometimes have negative consequences, as in the case of some isolated supplements," says Mortimer.
Hunter agrees, adding that "small changes can have a big impact in the quality if your food and the nutritional benefit you would get from that meal." It can also help us focus on getting those five (or ideally 10) a day, but ensuring a variety. "As healthy as broccoli is, you wouldn't eat 400 grams of it per day," says Hunter.
It can go the other way, too. Tannins found in tea, for example, can block the absorption of iron, so ideally tea shouldn't be drunk during mealtime.
Mortimer offers a word of caution: "While the pairings can certainly help people boost their nutrient intake, it's worth pointing out that it's not something we should be obsessing over. The research suggests it's more important to look at the diet as a whole, focusing on the age-old adage of including a wide variety of nutrient-dense foods in our diets."
1. Iron and Vitamin C
"Iron is an essential mineral and is particularly important for making haemoglobin, which carries oxygen around in the blood," says Mortimer. Iron deficiency is the most common nutrient deficiency in the UK, symptoms including tiredness and a heightened susceptibility to infections.
Luckily, it's fairly easy and very tasty to match the two. Here are some options:
- Beef and broccoli stir fry
- Red lentil and tomato soup
- Kale and citrus smoothie (kale contains Vitamin C but not in high enough levels, so an extra citrus boost is recommended)
- Fortified breakfast cereal with a small glass of orange juice
- Cavolo nero or kale with lemon
2. Calcium and Vitamin D
Similarly, Vitamin D helps our bodies absorb calcium. Mortimer explains: "While calcium has many functions, 99 per cent of it can be found in the bones and teeth where it forms their structure.
Poor mineralisation of the bones leads to conditions like osteoporosis and low levels of Vitamin D can cause rickets in children and osteomalacia (bone softening) in adults.
"Some foods like cheese contain both in a single package, but you may wish to consider pairing calcium-rich greens like kale or broccoli with oily fish, eggs, or Vitamin D enriched mushrooms."
- Fortified breakfast cereals with milk or fortified milk alternatives
- Cheese and Vitamin D enriched mushroom omelette
- Salmon and steamed greens
3. Fat soluble vitamins and fats
Fat soluble vitamins include A, D, E and K, and they are only absorbed in the intestine in the presence of fat. Vitamin A is the most likely deficiency after D. It is found in liver, milk, cheese, butter, carrots, sweet potatoes, dark green leafy veg, oranges and more.
"Recent research suggests that a fat source not only increases the biodiversity of fat soluble vitamins but may also help to convert beta carotene (found in carrots) into a useable source of Vitamin A for the body," says Mortimer.
Hunter recommends having olive oil with your salad, or an avocado or some nuts. "People are increasingly fat phobic," she explains. "but a small amount of dressing on a salad will not only help you absorb some of the nutrients from that salad, it also makes it taste much nicer!" The type of fat doesn't matter, and you could for example have some butter with your carrots, but that would add negatives like saturated fat.
- Sweet potato or carrots roasted in olive oil or butter
- Avocado or nuts in a green salad
- Orange slices and chopped nuts
4. Lycopene and fat
Lycopene is a carotenoid that's fat soluble. It's found in tomatoes, cherries, strawberries, papaya and most red fruit or veg. It's an antioxidant and there is current research into its potential to reduce the risk of cancer.
"While research is limited, single meal studies have suggested that eating fat alongside a lycopene-rich food may improve absorption of lycopene fourfold. Cooking can also increase lycopene availability," says Mortimer.
- Tomato salsa and avocado guacamole
- Olives and feta in a Greek salad
- Strawberries dipped in nut butter
- Red pepper and hummus
- Stewed tomato and olive oil pasta sauce